1. Field of the Invention
This invention pertains to propagation of ultrasonic frequencies from a thin diaphragm emitter. Specifically, the present invention relates to a device and method for indirectly generating a new sonic or subsonic compression wave by interaction of two ultrasonic signals having frequencies whose difference in value corresponds to the desired new sonic or subsonic compression wave frequencies.
2. State of the Art
Many attempts have been made to reproduce sound in its pure form. In a related patent application under serial number 08/684,311, a detailed background of prior art in speaker technology using conventional speakers having radiating elements was reviewed and is hereby incorporated by reference. FIG. 1 illustrates a graphic representation of a conventional audio speaker 10 using a moveable diaphragm 14. Diaphragm movement 18 is regulated by energy from a magnetic core which drives a stator 22 in a reciprocating manner within an annular recess of the coil. The conversion of electrical signal to sonic compression wave is developed by the variable current or voltage applied to the stator, resulting in a variable magnetic field which is attracted or repulsed with respect to the magnetic core. The diaphragm attached to the stator is displaced to mechanically reproduce the variable frequency and amplitude of the electrical signal in the form of a compression wave. Amplitude of the compression wave is primarily a function of the diameter of the diaphragm, and extent of orthogonal displacement. Physically, this corresponds to the volume of air being moved with each stroke of the speaker membrane.
The primary disadvantage with use of such conventional speakers is distortion arising from the mass of the moving diaphragm or other radiating component. Related problems arise from distortion developed by mismatch of the radiator element across the spectrum of low, medium and high range frequencies--a problem partially solved by the use of combinations of woofers, midrange and tweeter speakers.
Attempts to reproduce sound without use of a moving diaphragm include technologies embodied in parametric speakers, acoustic heterodyning, beat frequency interference and other forms of modulation of multiple frequencies to generate a new frequency. In theory, sound is developed by the interaction in air (as a nonlinear medium) of two ultrasonic frequencies whose difference in value falls within the audio range. Ideally, resulting compression waves would be projected within the air as a nonlinear medium, and would be heard as pure sound. Despite the ideal theory, general production of sound by acoustic heterodyning for practical applications has alluded the industry for over 100 years.
Specifically, a basic parametric or heterodyne speaker has not been developed which can be applied in general applications in a manner such as conventional speaker systems. Ultrasonic frequencies have comparatively small wave lengths and are generally characterized by nominal diaphragm displacement. This limited movement of the diaphragm or emitter membrane contributes to inadequate volume for the parametric output, as well as lack of extended range for projection of the resulting sonic waves generated by interference of the two ultrasonic frequencies well. It is not surprising that amplitude would be a problem in such a system where frequencies well in excess of 40,000 Hz tend to limit the excursion length for diaphragm displacement.
A brief history of development of the theoretical parametric speaker array will be helpful with respect to enhancing an appreciation for the confusion and inadequacies of prior efforts for increasing amplitude from an acoustic heterodyne system. For example, a general discussion of this technology is found in "Parametric Loudspeaker--Characteristics of Acoustic Field and Suitable Modulation of Carrier Ultrasound", Aoki, Kamadura and Kumamoto, Electronics and Communications in Japan, Part 3 Vol. 74, No.9 (March 1991). Although technical components and the theory of sound generation from a difference signal between two interfering ultrasonic frequencies is described, the practical realization of a commercial sound system was apparently unsuccessful. Note that this weakness in the prior art remains despite the assembly of a parametric speaker array consisting of as many as 1410 piezoelectric transducers yielding a speaker diameter of 42 cm. Virtually all prior research in the field of parametric sound has been based on the use of conventional ultrasonic transducers, typically of bimorph character. The rigid piezoelectric emitter face of such transducers has very little displacement, and is accordingly limited in amplitude.
U.S. Pat. No. 5,357,578 issued to Taniishi in October of 1994 introduced alternative solutions to the dilemma of developing a workable parametric speaker system. Hereagain, the proposed device comprises a transducer which radiates the dual ultrasonic frequencies to generate the desired audio difference signal. However, this time the dual-frequency, ultrasonic signal is propagated from a gel medium on the face of the transducer. This medium 20 "serves as a virtual acoustic source that produces the difference tone 23 whose frequency corresponds to the difference between frequencies f1 and f2." Col 4, lines 54-60. In other words, this 1994 reference abandons direct generation of the difference audio signal in air from the face of the transducer, and depends upon the nonlinearity of a gel medium to produce sound. This abrupt shift from transducer/air interface to proposed use of a gel medium reinforces the perception of apparent inoperativeness of prior art disclosures, at least for practical speaker applications.
Electrostatic emitters for ultrasonic wave generation have been applied in many areas of technology, but have equally limited diaphragm displacement. For example, ultrasonic emitters in range finder devices for cameras and distance measuring devices produce high frequencies, but with very little amplitude. U.S. Pat. No. 5,287,331 by Schindel illustrates devices which can generate extremely high frequencies up to 2 MHZ, but have an orthogonal displacement in micrometers. Because of the weakness of electrostatic forces, it is generally expected that diaphragm displacement will be nominal, as will be the resulting amplitude of ultrasonic or sonic output.
What is needed is a system that combines the substantial mechanical movement of conventional audio speakers which are magnetically driven, with the high frequency capacity of an electrostatic speaker which operates well within the ultrasonic frequency range.